A laser light source equipped with an ISO (Isolator) as an antireflection element is known as a laser light source used for laser processing and others. In such a laser light source, a Faraday rotation crystal to be used for forming the ISO is, for example, a TGG (Tb3Ga5O12) crystal or a TSAG (Tb3(ScAl)5O12) crystal having a positive thermooptic constant. Methods for controlling variation of beam propagation due to the thermal lens effect of these Faraday rotation crystals are disclosed, for example, in Patent Literatures 1-2 and Non Patent Literatures 1-2. Specifically, Patent Literature 1 and Non Patent Literatures 1-2 disclose the method of locating a DKDP (Deuterated Potassium Dihydrogen Phosphate) crystal having a negative thermooptic constant, or the like on the optical path, thereby compensating for the thermal lens effect of the TGG crystal or the TSAG crystal. Patent Literature 2 discloses a Faraday rotator capable of reducing the thermal lens effect, which can be applied to the ISO.